In wavelength division multiplexed systems, arrayed waveguide gratings (AWGs) are often used as optical multiplexers or demultiplexers. With reference to FIG. 1, an AWG 100 generally includes an array 102 of waveguides 103 typically having constant optical path length increments between adjacent waveguides 103, connected between an input free propagation region (FPR) 104 and an output FPR 106. When the AWG 100 operates as a wavelength demultiplexer, as depicted, input light including multiple wavelengths diffracting out of an input waveguide 108 or other input coupler into the input FPR 104 propagates through the input FPR 104 to illuminate the input ports of the array 102 of waveguides 103. After propagating through the array 102 of waveguides 103 and accumulating different optical phases in different ones of the waveguides 103 due to the different respective optical path lengths, light exiting the array 102 of waveguides 103 at their output ports is refocused in the output FPR 106, whereby light of different wavelengths constructively interferes, and thus refocuses, at different locations. A plurality of output waveguides 110 or other output couplers may be placed at the various foci so as to capture light of the respective wavelengths. To operate the AWG 100 as a multiplexer, the direction of light propagation through the AWG 100, and thus the roles of the FPRs 104, 106, can be reversed: light of multiple wavelengths is coupled from multiple respective waveguides 110 into the FPR 106 (which thereby functions as the input to the AWG 100) and dispersed in the FPR 106 to illuminate the array 102 of waveguides 103, and after propagating through the array 102 of waveguides 103, the now mixed-wavelength light from all of the arrayed waveguides 103 is refocused in the FPR 104 (now functioning as the output of the AWG), from where it exits into the waveguide 108.
When implemented in PICs, AWGs are susceptible to a number of factors that can affect their wavelength response, often resulting in the mismapping of wavelengths to output waveguides. For example, due to fabrication tolerances of PICs, the effective index of the waveguides in the array may not be controlled accurately enough to achieve an intended wavelength response. Fabrication-based deviations from the intended response are especially likely if the waveguide dimensions are small, if the AWG waveguide core is in a deposited layer for which thickness control is poor, or if the refractive index of the waveguide core is dependent upon material growth or deposition conditions. In addition to these problems, the effective index of the waveguides in the array, and thus the wavelength response of the AWG as a whole, varies as a function of temperature. This effect is particularly pronounced where waveguide core materials having a large thermooptic coefficient, such as silicon, are used, and tends to limit the temperature range over which the AWG can be used. One approach to reducing undesirable wavelength shifts of the AWG response due to fluctuations in the ambient temperature, and/or to compensating for fabrication-based deviations from the desired wavelength response, involves actively controlling the temperature of the AWG, which, however, requires a significant amount of power, rendering the PIC less efficient.